The present invention relates to a stencil printer for printing an image on a paper sheet or similar recording medium with a master perforated by a thermal head or similar heat generating device and wrapped around a print drum.
A digital thermal stencil printer using a thermal head is conventional. The thermal head includes fine heat generating elements arranged in the main scanning direction. While a stencil is conveyed along a preselected path, current in the form of pulses is selectively fed to the heat generating elements contacting the stencil in accordance with image data, thereby perforating the stencil. The perforated stencil (master hereinafter) is wrapped around a hollow cylindrical, porous print drum. A press drum pressed against the print drum is rotated to cause ink to be transferred from the print drum to a paper sheet via the perforations of the master, so that an image is printed on the paper sheet.
Usually, the press roller is movable into an out of contact with the print drum in synchronism with the rotation of the print drum. Specifically, when the ink should be transferred from the print drum to the paper sheet, the press roller is pressed against and rotated by the print drum. When the press roller is being moved toward the print drum without being rotated, the former is rotating in the opposite direction relative to the latter. This brings about a problem that when the press roller contacts the non-perforated portion (non-image portion) of the master, it pulls the master in the direction opposite to the direction of rotation of the print drum, tearing the master or pulling it out of a master clamper. Further, on contacting the paper sheet, the press roller displaces or tears the paper sheet in addition to the master. In any case, a high quality image is not achievable with the above-described configuration.
In light of the above, a mechanism has been proposed for causing, before the press roller is moved toward the print drum, the roller to rotate in the direction in which the print drum rotates during printing. This kind of mechanism is disclosed in, e.g., Japanese Utility Model Laid-Open Publication No. 60-145069 and Japanese Patent Laid-Open Publication Nos. 6-24117 and 10-305649. Let the above rotation of the press roller be referred to as preliminary rotation hereinafter.
Specifically, the above Laid-Open Publication No. 60-145069 shows in FIG. 1 thereof a first preliminary rotation mechanism in which a print drum 1 is provided with a preliminary rotation rubber 23. In this mechanism, a press roller 18 contacts the preliminary rotation rubber 23 while moving toward the print drum 1 and is caused to rotate thereby. The same document shows in FIGS. 2 and 3 thereof a second preliminary rotation mechanism including a support arm 7 supporting a press roller 8 and provided with a device for rotating the press roller 8. The second mechanism rotates the press roller 8 while moving it toward the print drum 1.
Laid-Open Publication No. 6-24117 teaches a third preliminary rotation mechanism including drive source assigned to a press roller in addition to a drive source assigned to a print drum, and a device for controlling the output torque of the drive source in accordance with the position of the press roller relative to the print drum. The third mechanism also rotates the press roller while moving it toward the print drum. Laid-Open Publication No. 10-305649 shows in, e.g., FIG. 2 thereof a fourth preliminary rotation mechanism in which conveying means 17a for conveying a paper sheet causes a press roller 9a being moved toward a print drum la to rotate. However, a problem with the first preliminary rotation mechanism is that every time the press roller 18 contacts the preliminary rotation rubber 23, it presses the print drum 1 via the rubber 23 and thereby deforms the drum 1 while adversely effecting the positional accuracy of the drum 1. Another problem with this mechanism is that the print drum 1 is not applicable to a stencil printer using a press drum, as taught in, e.g., Japanese Patent Laid-Open Publication No. 11-48595. The second and third preliminary rotation mechanisms each need a sophisticated configuration. The fourth preliminary rotation mechanism makes the rotation speed of the press roller 9a unstable because it causes the press roller 9a to rotate while moving it toward the print drum 1a. 
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 64-18682 and 5-229243.
It is therefore an object of the present invention to provide a stencil printer capable of effecting the preliminary rotation of a press roller at a stable, accurate speed with a relatively simple configuration while freeing a master, a paper sheet and a print drum from loads.
In accordance with the present invention, a stencil printer of the type including a print drum around which a master is to be wrapped has a press roller for pressing a paper sheet or similar recording medium against the master wrapped around the print drum. A press roller displacing device moves the press roller between an operative position where it presses the recording medium against the master and an inoperative position where it is spaced from the print drum. A press roller driving device causes the press roller held at the inoperative position to rotate, at a position where the roller faces the print drum, in the same direction as the direction in which the drum rotates during printing. The press roller driving device causes the press roller to rotate only when the roller is held at the inoperative position.